This invention is related to the field of photodiodes. More particularly, it is related to silicon photodiodes useable in high-speed telecommunications applications.
Silicon photodiodes are semiconductor devices responsive to high-energy particles and photons. Photodiodes operate by absorption of photons or charged particles and generate a flow of current in an external circuit, proportional to the incident power. Photodiodes can be used to detect the presence or absence of minute quantities of light. Planar diffused silicon photodiodes (also known as PIN diodes) are simply P-N junction diodes. A P-N junction can be formed by diffusing either a P-type impurity (anode), such as Boron, into a N-type bulk silicon wafer, or a N-type impurity (cathode), such as phosphorous, into a P-type bulk silicon wafer. The diffused area defines the photodiode active areaxe2x80x94that region of the photodiode sensitive to incident radiation.
To form an ohmic contact another impurity diffusion into the backside of the silicon wafer is necessary. The impurity is an N-type for a diode with a P-type active area and the impurity is P-type for a diode with an N-type active area. The contact pads are typically deposited on the front active area in defined areas, and on the backside, completely covering the device. The active area is covered with an anti-reflection coating to reduce the reflection of the light for a specified predefined wavelength. The non-active area on the top is covered with a thick layer of silicon oxide (SiO2). By controlling the thickness of the bulk substrate, the speed and response of the photo diode can be controlled. Photo diodes, when biased, must be operated in the reverse bias mode, i.e., a relatively negative voltage applied to the anode and a relatively positive voltage applied to the cathode.
FIG. 1 illustrates a conventional silicon photodiode 10 built on an N-type substrate 12. The front side 14 of the diode 10 includes an active photo receptor area 16 coated with an anti-reflection coating 18 and a non-active area 20 coated with a layer of SiO2 22. A P+ diffusion region 24 is formed in the substrate in the area of the active photoreceptor area. Between the P+ diffusion region 24 and the substrate 12 exists a depletion region 26. A metallization layer 28 is formed in contact with an N+ diffusion region 30 on the backside 32 of the silicon wafer 34. A first contact 35 forms an electrical contact with P+ diffusion region 24. Metal layer 28 forms a second contact.
FIG. 2 illustrates a conventional photodiode 36 built on a P-type substrate 38. The structure is similar to that of the diode of FIG. 1 except that current flow is reversed.
Silicon is a semiconductor with a band gap energy of 1.12 eV at room temperature. This is the gap between the valence band and the conduction band. At absolute zero temperature the valence band is completely filled and the conduction band is vacant. As the temperature increases, the electrons become excited and escalate from the valence band to the conduction band by thermal energy. The electrons can also be escalated to the conduction band by particles or photons with energies greater than 1.12 eV, which corresponds to wavelengths shorter than 1100 nm. The resulting electrons in the conduction band are free to conduct current.
Due to the concentration gradient, the diffusion of electrons from the N-type region to the P-type region and the diffusion of holes from the P-type region to the N-type region, develops a built-in voltage across the junction. The inter-diffusion of electrons and holes between the N and P regions across the junction results in a region with no free carriers. This is the depletion region. The built-in voltage across the depletion region results in an electric field with a maximum at the junction and no field outside of the depletion region. Any applied reverse bias adds to the built-in voltage and results in a wider depletion region. The electron-hole pairs generated by light are swept away by the electric field of the depleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially down with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths (FIG. 3). Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all.
A silicon photodiode can be represented by a current source in parallel with an ideal diode (FIG. 4). The current source represents the current generated by the incident radiation, and the diode represents the p-n junction. In addition, a junction capacitance (Cj) and a shunt resistance (RSH) are in parallel with the other components. Series resistance (RS) is connected in series with all components in this model.
Shunt Resistance, RSH 
Shunt resistance is the slope of the current-voltage curve of the photodiode at the origin, i.e. V=0. Although an ideal photodiode should have a shunt resistance of infinite, actual values range from 10 s to 1000 s of Mega ohms. Experimentally it is obtained by applying xc2x110 mV, measuring the current and calculating the resistance. Shunt resistance is used to determine the noise current in the photodiode with no bias (photovoltaic mode). For best photodiode performance the highest shunt resistance is desired.
Series Resistance, RS 
Series resistance of a photodiode arises from the resistance of the contacts and the resistance of the undepleted silicon (FIG. 1). It is given by:                               R          s                =                                                            (                                                      W                    s                                    -                                      W                    d                                                  )                            ⁢              ρ                        A                    +                      R            c                                              (                  EQ          .                      xe2x80x83                    ⁢          1                )            
where Ws is the thickness of the substrate, Wd is the width of the depleted region, A is the diffused area of the junction, xcfx81 is the resistivity of the substrate and Rc is the contact resistance. Series resistance is used to determine the linearity of the photodiode in photovoltaic mode (no bias, V=0). Although an ideal photodiode should have no series resistance, typical values ranging from 10 to 1000 ohms are measured.
Junction Capacitance, Cj 
The boundaries of the depletion region act as the plates of a parallel plate capacitor (FIG. 1). The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. Furthermore, the capacitance is dependent on the reverse bias as follows:                               C          j                =                                            ϵ              Si                        ⁢                          ϵ              o                        ⁢            A                                              2              ⁢                              xe2x80x83                            ⁢                              ϵ                Si                            ⁢                              ϵ                o                            ⁢              μ              ⁢                              xe2x80x83                            ⁢                              ρ                ⁢                                  (                                                            V                      A                                        +                                          V                      bi                                                        )                                                                                        (                  EQ          .                      xe2x80x83                    ⁢          2                )            
where ∈o=8.854xc3x971014 F/cm, is the permitivity of free space, ∈Si=11.9 is the silicon dielectric constant, xcexc=1400 cm2/Vs is the mobility of the electrons at 300xc2x7K, xcfx81 is the resistivity of the silicon, Vbi is the built-in voltage of silicon and VA is the applied bias. FIG. 5 shows the dependence of the capacitance on the applied reverse bias voltage. Junction capacitance is used to determine the speed of the response of the photodiode.
Rise/Fall Time and Frequency Response, t/t/f3dB 
The rise time and fall time of a photodiode is defined as the time for the signal to rise or fall from 10% to 90% or 90% to 10% of the final value respectively. This parameter can be also expressed as frequency response, which is the frequency at which the photodiode output decreases by 3 dB. It is roughly approximated by:                               t          r                =                  0.35                      f                          3              ⁢              dB                                                          (                  EQ          .                      xe2x80x83                    ⁢          3                )            
There are three factors defining the response time of a photodiode:
1. tDRIFT, the charge collection time of the carriers in the depleted region of the photodiode.
2. tDIFFUSED, the charge collection time of the carriers in the undepleted region of the photodiode.
3. tRC, the RC time constant of the diode-circuit combination.
tRC is determined by tRC=2.2 RC, where R is the sum of the diode series resistance and the load resistance (Rs+Rl), and C is the sum of the photodiode junction and the stray capacitances (Cj+Cs). Since the junction capacitance (Cj) is dependent on the diffused area of the photodiode and the applied reverse bias (Equation 2), faster rise times are obtained with smaller diffused area photodiodes, and larger applied reverse biases. In addition, stray capacitance can be minimized by using short leads, and careful layout of the electronic components. The total rise time is determined by:
tR={square root over (tDRIFT2+tDIFFUSED2+tRC2)}xe2x80x83xe2x80x83(EQ. 4)
Generally, in photovoltaic mode of operation (no bias), rise time is dominated by the diffusion time for diffused areas less than 5 mm2 and by RC time constant for larger diffused areas for all wavelengths. When operated in photoconductive mode (applied reverse bias), if the photodiode is fully depleted, such as fiber optic series, the dominant factor is the drift time. In non-fully depleted photodiodes, however, all three factors contribute to the response time.
Most high-speed photodiodes are now made with Indium Galium Arsenide (InGaAs) substrates due to the need for high performance in the 1.2-1.6 xcexcm wavelength range where attenuation is lowest in silicon-based fiber optic cables. Silicon photodiodes exhibit useable sensitivities in the 0.750-0.900 xcexcm range and, as a result, have not yet found significant application in long-haul telecommunications systems.
FIG. 6 illustrates the attenuation of light as a function of wavelength in silicon fiber optic material. Total attenuation shown is the sum of absorption and scattering losses. At 0.8 xcexcm, attenuation is about 2-3 dB/km and scattering losses. While at 1.3 xcexcm, attenuation is only about 0.35 dB/km.
Recently, however, as fiber optic telecommunications systems have found application in relatively short-haul communications systems, the lower cost of silicon photodiode makes them attractive in so-called campus and metropolitan area networking where transmission distances are less than a few km.
While the attenuation problems due to operation in the 750 nm-900 nm range is, therefore, capable of being overcome, today""s telecommunication systems need to be able to operate at high speeds in excess of 1 GB/sec with low voltage bias. Accordingly, a high-speed ( greater than 1 GB/sec) silicon photodiode operable at a bias of about 3.3 volts or less would be desirable.
In accordance with the present invention, novel techniques are used to reduce the response time of a silicon photodiode by reducing the drift time of electron-hole pairs within the bulk of the wafer on which the silicon photodiode is fabricated.
A high-speed silicon photodiode and method of manufacture include a first layer of silicon having thickness in a range of about 125 xcexcm to about 550 xcexcm. A second layer of silicon has a thickness in a range of about 3 xcexcm to about 16 xcexcm and a resistivity of at least about 500 ohm-cm. This first layer is doped with a second type of impurity. In an alternative aspect, a high-speed silicon photodiode and method of manufacture includes a silicon wafer doped with a first type of impurity. On a first side of the wafer a doping of a second type is applied in an active area of a photodiode. On the reverse of the wafer a volume of silicon is etched away and the resulting trench is coated with a conductor. The wafer may also exhibit a high resistivity of at least about 500 ohm-cm. In each aspect, a reverse bias not exceeding about 3.3 volts permits operation with a frequency response of at least 750 MHz.